Chemical Formulas and Representations Summary

Molecular representations and models

  • Molecular formula: uses chemical symbols with subscripts to show the types and numbers of atoms in a molecule; a subscript is used only when more than one atom of a given type is present. Used as an abbreviation for the compound’s name. Example: ext{CH}_4 for methane.
  • Structural formula: conveys the same atom types and counts as the molecular formula but also shows how atoms are connected; lines indicate bonds.
  • Ball-and-stick model: shows geometric arrangement of atoms; not to scale.
  • Space-filling model: shows relative sizes of atoms.
  • Methane representations: (a) molecular formula, (b) structural formula, (c) ball-and-stick model, (d) space-filling model.

Subscripts and formulas: meanings and differences

  • A subscript after a symbol (e.g., ext{H}_2) counts atoms in a molecule; a coefficient (e.g., 2H) indicates multiple atoms or entities.
  • Distinction:
    • ext{H}_2: a diatomic molecule of hydrogen (two hydrogen atoms bonded together).
    • 2 ext{H}: two separate hydrogen atoms (not bonded as a unit).
    • 2 ext{H}_2: two molecules of diatomic hydrogen.
  • This shows that symbols like H, H2, 2H, and 2H2 represent different entities.

Empirical vs molecular formulas

  • Empirical formula: the simplest whole-number ratio of the types of atoms in a compound.
  • Molecular formula: shows the actual number of each type of atom in a molecule.
  • Relationship: the molecular formula is always a whole-number multiple of the empirical formula.
  • Example: titanium dioxide has empirical formula ext{TiO}_2 (ratio 1 Ti : 2 O).

From empirical to molecular formulas and back

  • In many cases, the molecular formula is determined experimentally from both empirical formula and molecular mass.
  • If a compound’s empirical formula is known, multiplying by an integer gives the molecular formula.

Benzene: a key example

  • Benzene molecular formula: ext{C}6 ext{H}6
  • Its empirical formula: ext{CH} (for every carbon atom there is one hydrogen).
  • Benzene can be represented as structural, ball-and-stick, or space-filling models.

Acetic acid: a detailed example

  • Molecular formula: ext{C}2 ext{H}4 ext{O}_2.
  • Empirical formula: ext{CH}_2 ext{O}.
  • Atom ratio in empirical formula: 2:4:2 for ext{C:H:O}.
  • Simplified ratio: dividing by the lowest common denominator 2 gives 1:2:1, so empirical formula is ext{CH}_2 ext{O}.
  • Acetic acid can be depicted as structural, ball-and-stick, or space-filling models.

Quick takeaways

  • Different representations highlight different aspects: composition (molecular formula), connectivity (structural formula), and geometry (ball-and-stick, space-filling).
  • Subscripts vs coefficients convey fundamentally different ideas; be careful to distinguish them.
  • Empirical formulas give the simplest atom ratio; molecular formulas give the actual atoms in a molecule; the former scales to the latter by a whole-number factor.
  • Many elements exist as diatomic molecules (e.g., ext{H}2, ext{O}2, ext{N}2, ext{F}2, ext{Cl}2, ext{Br}2, ext{I}2); sulfur commonly forms ext{S}8 rings.